1. Field of the Invention
The present invention relates to a sense amplifier circuit which is, for example, used in a MIS static random access memory (RAM).
2. Description of the Prior Art
In general, a MIS static RAM cell comprises a bistable flip-flop circuit which uses four or six transistors per bit. That is, the memory cell has a pair of driver transistors which are cross-coupled to each other, a pair of load resistors or load transistors, and a pair of transfer transistors connected to one word line and to one pair of bit lines. In this memory cell, only one of the driver transistors is turned on to correspond to memory data "1" or "0". In order to read the cell, the transfer transistors are turned on by changing the potential of the word line and the data on the driver transistors is transferred to the bit lines. In this memory, a sense amplifier circuit is provided for sensing and amplifying a small difference in potential between the bit line pair.
One conventional sense amplifier circuit used in a MIS static RAM has a differential amplifier for sensing and amplifying a small difference in potential between a bit line pair, and a pull-down circuit for generating a reference potential to the differential amplifier. In this case, the pull-down circuit is controlled by a power supply or a voltage which is in response to the power supply. However, in this conventional sense amplifier circuit, when the potential of the power supply is increased, the potentials of the bit line pair are also increased, while the reference potential for the differential amplifier is unchanged or decreased. As a result, the difference between the potentials of the bit line pair and the reference potential is increased, which deteriorates the sensing operation of the sense amplifier circuit. In other words, the sensing operation is susceptible to fluctuation in the power supply. In particular, in a high-integrated memory wherein the cell area thereof is small, the conductance gm of the driver transistors is small, so that the difference in potential between the bit line pair is very small. As a result, the sensing operation is very susceptible to fluctuation in the power supply.